Control ratio
Control ratio
(OP)
What is typical control ratio of a control valve for HVAC application with metal to metal sealing? I understand that a control valve with metal to metal sealing should have higher range ability compared to the one with soft sealing.





RE: Control ratio
.. I am afraid you have to be more specific....
RE: Control ratio
RE: Control ratio
Lets say that we have two control valves with linear characteristic, one with metal to metal sealing and one with soft sealing. It is assumed that soft sealing element will cause an initial jump at the very begining of the valve`s opening.
What about metal to metal sealing? Does this valve in above mentioned case performs better?
RE: Control ratio
RE: Control ratio
I don't know why you assume soft seats cause an intial jump in valve position. That is very much a property of the valve design. Maybe double-offset butterfly valves with TFE seats, but that's about it. I have seen a LOT more metal seated valves with poor positioner calibration that had a jump at initial opening.
I have had success in using eccentrically rotating plug valves for this application without resorting to the 1/3-2/3 split. The ERP valve has greater than 100:1 rangeability. With better than class IV leakage, it barely leaks enough to keep the pipes warm...and thermal cycling the pipes is noisy as they pop and creak as the temp changes. THermal cycling the pipes causes fatigue-related problems, too.
Pay attention to the steam trap installation also. Poorly installed steam traps lead to slugs of water in the line and steam hammer in the pipes....which is really "condensate hammer"
RE: Control ratio
This will exist in all ball and butterfly valves to some extent or other.
B
RE: Control ratio
You won't see a difference in jumping between metal and soft seated globe valves.
Rangeability varies with the valve type and in general, you can say that metal seated valves tend to be more tolerant to throttling close to the seat. However, you really need to look into the specifics of the valve to be sure of its capabilities.
RE: Control ratio
Hmmm Unseating and breakout of metal or soft is another word for 'jump' sorry if the terminology was a little simplified.
Globe valves don't have the same sealing characteristics as butterfly and ball.
Metal seated valves are more tolerant to working near the closed position as they have a far greater resistance to errosion when higher flows exist as the valve closes.
B